CHICAGO (CBS) -- The only thing taller in Chicago than the Willis Tower is the amount of money its new owners are paying for it.

New York-based Blackstone Group LP, has reached an agreement to buy the iconic skyscraper for $1.3 billion, according to Marketwatch.

The deal for the building once called the Sears Tower would be the highest price ever paid for an American office building outside New York.

A spokesman for the company confirmed the purchase, but would not discuss terms of the transaction.

Blackstone, a private equity firm, is expected to focus on the retail aspect of the nation's second-tallest building, by upgrading the Skydeck – home to the "Ledge," a set of glass and steel boxes extending from the top of the building – and other attractions to bring in more money from tourists.

Approximately 1.6 million people visited the 110-story tower last year.

Bruce Miller of Jones Lang LaSalle says Blackstone is a smart investor, likely to focus on filling up the 15 percent vacancy rate and adding retail.

"Chicago's got a great market here now and a lot of demand for office space," he said.

The building last sold for $840 million in 2004 to a group of real estate investors, who sold the naming rights in 2009 to Willis Group Holdings, a British insurance broker that leases office space in the skyscraper.